1. Field of the Invention
The present invention relates to an improved system for reducing sulfur compound content in a hydrocarbon gas stream. More particularly, the desulfurizing system described herein may be used to reduce the presence of inorganic and organic sulfur compounds in the gas stream to levels acceptable for, e.g., subsequent reforming of the hydrocarbon.
2. Discussion of the Prior Art
The partial oxidation of hydrocarbons, for example, methane, in the presence of a catalyst is an attractive route for the preparation of mixtures of carbon monoxide and hydrogen, known in the art as synthesis gas. The partial oxidation of a hydrocarbon is an exothermic reaction and, in the case in which methane is the hydrocarbon, proceeds by the following reaction (1):2CH4+O2→2CO+4H2  (1)
Another important source of hydrogen and synthesis gas is derived from steam reforming a hydrocarbon such as methane. Catalyzed steam reforming is endothermic and in the case in which methane is the hydrocarbon, proceeds by the following reaction (2):CH4+H2O→CO+3H2  (2)
It is well known and desirable to reduce the level of gaseous sulfur compounds such as hydrogen sulfide (H2S), carbonylsulfide (COS), mercaptans (R—SH), and sulfides (R1—S—R2) from hydrocarbon streams prior to reforming the hydrocarbon stream into useful gaseous components such as by either of reactions (1) and (2) above. Many applications, e.g., fuel cells, require that the gaseous sulfur compounds in a gas stream (e.g., naphtha, liquid petroleum gas (LPG), town gas, etc.) be reduced to as low a level as practicable in order to avoid polluting the environment or poisoning (i.e., deactivating) catalysts such as steam reforming catalysts, water-gas shift catalysts, etc. Fuels, such as natural gas, gasoline, diesel fuel, naphtha, fuel oil, LPG and like hydrocarbon fuels may not be useful as a process fuel source due to the existence of relatively high levels of naturally-occurring complex organic sulfur compounds, or sulfur compounds added as odorants, such as mercaptans and sulfides.
Desulfurization of hydrocarbon streams is particularly beneficial for hydrogen generation and use thereof in fuel cells. Conventional fuel processing systems used with stationary fuel cell power plants include a thermal steam reformer, such as that described in U.S. Pat. No. 5,516,344. In such a fuel processing system, sulfur is removed by conventional hydrodesulfurization techniques, which typically rely on a certain level of recycle as a source of hydrogen for the process. The recycle hydrogen combines with the organic sulfur compounds to form hydrogen sulfide within a catalytic bed. The hydrogen sulfide is then removed using a zinc oxide bed to form zinc sulfide. A general hydrodesulfurization process is disclosed in detail in U.S. Pat. No. 5,292,428. There are many such prior art processes involving hydrogenation desulfurization in which the sulfur compounds in the fuel stream are decomposed by hydrogenolysis at temperatures of, e.g., 350 to 400° C. in the presence of e.g., Ni—Mo or Co—Mo catalysts and thereafter the resultant H2S is then absorbed on a bed of ZnO at temperatures of, e.g., 300 to 400° C. However, in these processes, the level of the H2S remaining in the treated stream is often too high, e.g., 1 ppmV and higher. It is well known low levels of gaseous sulfur compounds will deactivate steam reforming nickel-based catalysts. Additionally, to remove the sulfur compounds from the gas being treated, hydrogen must be provided to the gas stream. In the case where the source of hydrogen is product gas in the form of recycle, this will reduce the overall efficiency of the power forming process.
Hydrogen sulfide has also been removed from gas streams by passing the gas stream through a liquid scrubber, such as sodium hydroxide, potassium hydroxide, or amines. Liquid scrubbers are large and heavy, and require large chemical inventories. Clean-up of the product gas is often needed to prevent carryover of the base scrubbing chemicals.
Still another process for removing sulfur compounds from hydrocarbon gas streams involves passing the gas stream directly through an adsorbent, which captures the sulfur species. Although the adsorption process operates at moderate temperatures and atmospheric pressure, a large inventory of adsorbent is needed. For natural gas, large volumes of one or more adsorbents are required for reasonable time on stream, e.g., one year, typically up to 20 liters total volume for a 2.5 kilowatt electric (kWe) average output fuel cell. Furthermore, natural gas composition variability makes choosing the appropriate adsorbents and bed sizes complicated and costly. For LPG, desulfurization by adsorption is particularly difficult due to the potentially high sulfur concentrations in LPG and adsorption interferences from LPG hydrocarbons.